Biosynthesis of rat liver cytochrome P-450 in mitochondria-associated rough endoplasmic reticulum and in rough microsomes in vivo

Iron and porphyrin trafficking in heme biogenesis

Iron is an essential element for diverse biological functions. In mammals, the majority of iron is enclosed within a single prosthetic group: heme. In metazoans, heme is synthesized via a highly conserved and coordinated pathway within the mitochondria. However, iron is acquired from the environment and subsequently assimilated into various cellular pathways, including heme synthesis. Both iron and heme are toxic but essential cofactors. How is iron transported from the extracellular milieu to the mitochondria? How are heme and heme intermediates coordinated with iron transport? Although recent studies have answered some questions, several pieces of this intriguing puzzle remain unsolved.

Cytosolic persistence of mouse brain CYP1A1 in chronic heme deficiency

Previous work has demonstrated that the function of extrahepatic cytochrome P450 CYP1A1 is dependent on the availability of heme. CYP1A1 is involved in the activation of polyaromatic hydrocarbons. In the present study we used a transgenic mouse model with chronic impairment of heme synthesis - female porphobilinogen deaminase-deficient (PBGD-/-) mice - to investigate the effects of limited heme in untreated and beta-naphthoflavone (beta-NF)-treated animals on the function of CYP1A1 in brain. The heme content of PBGD-/- mice was diminished in the liver and brain compared to wild types. In the liver, partial heme deficiency led to less potent induction of CYP1A1 mRNA after beta-NF treatment. In the brain, CYP1A1 protein was detected not only at the endoplasmic reticulum (ER), but also in the cytosol of PBGD-/- mice. Furthermore, 7-deethylation of ethoxyresorufin, an indicator of CYP1A1 metabolic activity, could be restored by heme in cytosol of PBGD-/- mouse brain. Independent of the genotype, we found only one cyp1a1 gene product, indicating that the cytosolic appearance of CYP1A1 most likely did not originate from mutant alleles. We conclude that heme deficiency in the brain leads to incomplete heme saturation of CYP1A1, which causes its improper incorporation into the ER membrane and persistence in the cytosol. It is suggested that diseases caused by relative heme deficiency, such as hepatic porphyrias, may lead to impaired hemoprotein function in brain.

Cytochrome P450 in rat astrocytes in vivo and in vitro: intracellular localization and induction by phenytoin

Cytochrome P450IIB1,2 (nomenclature according to Nelson et al., DNA Cell Biol 12:1-51, 1993 and Volk et al., Neuroscience 42:215-235, 1991) immunoreactivity (P450-IR) is associated with astrocytes both in vivo and in vitro. Although they are unevenly distributed throughout the brain with a preference for phylogenetically elder parts, no significant differences between astrocytes prepared from different brain regions were observed in astrocyte cultures. The percentage of strongly immunoreactive astrocytes decreased from 40% after 7 days in culture to 15% after 21 days. Essentially all astrocytes have a low but significant P450-IR within this interval. Preembedding immunoelectron microscopy revealed peroxidase reaction products on the endoplasmic reticulum and on the outer membranes of mitochondrial and nuclear envelopes. Phenytoin (1 microM) added to the medium for 7 days significantly (1.22-fold) increased the amount of total P450 in astrocyte homogenates as measured by spectrophotometry. Considerably more immunoreactive cells (1.5-fold) were found in treated cultures than in controls.

Preferential heme transport through endoplasmic reticulum associated with mitochondria in rat liver

The transport of de novo synthesized protoheme into the conventional microsomal fraction and endoplasmic reticulum associated with mitochondria (MAER) was studied by injecting amino[14C]levulinic acid (ALA) into phenobarbital-treated rats to evaluate the role of MAER in the trafficking of heme between mitochondria and endoplasmic reticulum. In mitochondria, the specific radioactivity of the radiolabeled heme reached a maximum level at 4 min after the injection of 14C-ALA. The specific radioactivity in cytosol was about 2-fold lower than that in microsomes, suggesting that the cytosolic pathway of the heme transport from mitochondria to endoplasmic reticulum is not predominant, because the specific radioactivity of heme in cytosol should be higher than that in microsomes if heme is transported mainly through cytosol. MAER showed higher specific radioactivity than the conventional microsomal fraction up to 4 min and thereafter the specific radioactivities in MAER and the conventional microsomal fraction became nearly the same. The extents of decrease in cytochrome P-450 and the radioactivity in microsomes by the treatment with allylisopropylacetamide which destroyed cytochrome P-450 but not cytochrome b5, were essentially the same, suggesting that most of the radiolabeled heme in microsomes was incorporated into cytochrome P-450. These results suggest that MAER is a preferential site for the protoheme transport from mitochondria to endoplasmic reticulum.

Reversibility of rat liver cirrhosis by medroxyprogesterone acetate

The possible effects of a synthetic progesterone, medroxyprogesterone acetate (MPA), on carbon tetrachloride/phenobarbital (CCl4/PB)-induced rat liver injury were studied by morphological methods. CCl4/PB-treated rats showed extensive liver fibrosis consisting of procollagen type III aminoterminal propeptide-positive strands and fibres with concomitant extensive basement membrane deposits and fibronectin synthesis. MPA treatment after CCl4/PB-induced liver damage reduced alterations in cytoplasmic organelles, inflammation and hemorrhages and reversed the fibrosis, mostly around individual liver cells, possibly due to the normalization of cellular structure and function with a decrease in fibronectin deposits.

Hepatic mitochondrial coumarin 7-hydroxylase: comparison with the microsomal enzyme

The specific activity of cytochrome P450-linked coumarin 7-hydroxylase (COH) of hepatic mitoplasts from DBA/2N mice is up to 55% as great as the microsomal activity. According to Western blot and immunodiffusion analysis and inhibition studies with anti-P450Coh and metyrapone, the mitoplastic P450Coh had the same molecular weight and immunochemical and catalytic properties as the corresponding microsomal enzyme. The inducibility of the two proteins by pyrazole and their genetic regulation, as studied with DBA/2N and AKR/J mice, appears to be similar. However, the mitochondrial electron transfer system was not able to support the COH activity of reconstituted microsomal P450Coh although the enzyme was fully active with the microsomal NADPH-cytochrome P450 reductase. This indicates some differences between the two proteins with respect to their interaction with the electron transfer system. This was confirmed by the ability of anti-adrenodoxin reductase antibody to effectively inhibit the mitoplastic COH but not the COH reconstituted with purified microsomal P450Coh and NADPH-P450 reductase. We have previously found that P450Coh does not react with anti-P450b or anti-P450c antibodies, which recognize respective isoforms in rat liver mitoplasts. While P450Coh from microsomes and mitoplasts possess a number of properties in common, the mitoplast P450Coh represents a new subspecies of mitochondrial P450. Some characteristics of mitoplast P450Coh may be the result of post-translational modifications necessary for processing and translocation into the mitochondria.

Demonstration of a transcellular vesicle pathway for biliary excretion of inulin in rat liver

We tested the hypothesis that the blood to bile transport of hydrophilic inert nonelectrolytes such as inulin is mediated in part by a transcellular pathway involving endosomelike vesicles (transcytosis). Forty minutes after intravenous injection of [3H]methoxyinulin into renal pedicle ligated rats, 0.8% of the radioactivity was recovered in liver homogenate and 85% +/- 3.6% of this radioactivity was associated with membrane bound vesicles. Subcellular fractionation studies and electron microscopy confirmed this association. If rats were treated with taurochenodeoxycholic acid, 5 mumol/100 g body wt, the hepatocellular uptake of [3H]methoxyinulin increased approximately twofold and [3H]methoxyinulin was again recovered in small subcellular vesicles. Furthermore, taurochenodeoxycholic acid also stimulated the biliary excretion of [3H]methoxyinulin, which peaked in bile at 20 min. Taurochenodeoxycholic acid had similar effects on the biliary excretion of horseradish peroxidase, a protein known to be transported from blood to bile by membrane vesicles. Thus under the conditions of these experiments, the dihydroxy bile acid taurochenodeoxycholic acid can stimulate the rate of vesicle-dependent transcellular transport into bile. If inulin clearance represents a maximal estimate of this process, only 6%-8% of total bile production in the rat under basal conditions would be mediated by vesicle-mediated transcytosis.